1. Field of the Invention
The present invention relates to a fastening element including a shaft, rotation-transmitting means provided at one end of the shaft, the shaft having a free end remote from the rotation-transmitting means, and a tapping thread carried by the shaft and having at least one screw thread with a thread pilot, a section of the at least one screw thread that adjoins the thread pilot, having a plurality of following one another thread steps having different cross-sections, with the thread steps having respective cutting edge-forming end surfaces aligned, at least partially, in a direction of the thread pilot.
2. Description of the Prior Art
Fastening elements of the type described above such as, e.g., concrete screws or sleeves with an internal thread, are screwed in a preliminary formed borehole with a setting tool such as, e.g., a tangential impact screwdriver, with the tapping thread of the fastening element tapping a counter-thread in the borehole wall for anchoring the fastening element in the borehole.
Mineral constructional components, such as concrete or brickwork into which the described above fastening elements are set, are brittle materials. During tapping of the counter-thread, material spalling takes place, which increase the size of the produced counter-thread above the necessary value. Because of this increase, the loads, which are transmitted by the set fastening element, decrease.
The deformations of the fastening element and of the constructional component, which take place during a setting process necessitate a high screw-in torque and, therefore, a correspondingly formed setting tool.
German Publication DE 101 13 946 A1 discloses a fastening element formed as a concrete screw and having rotation-transmitting means that cooperates with a setting tool and a tapping thread-carrying shaft having a free end remote from the rotation-transmitting means. The tapping thread has a screw thread that is provided, in the region of the shaft free end, with cutting notches.
The drawback of the fastening element disclosed in DE 101 13 946 A1 consists in that the counter-thread in the constructional component is produced by crushing and not by cutting. Therefore, during the formation of the counter-thread material, spalling occurs. Thereby, the produced, in the damaged region, undercut substantially limits the load transmission.
German Utility Model DE 91 03 773 U discloses a fastening element which is formed as a self-drilling screw and has a shaft, rotation transmitting means provided at one end of the shaft, the shaft having a free end remote from the rotation-transmitting means, and a tapping thread carried by the shaft. The tapping thread has screw thread with a thread pilot. The screw thread has, proceeding from the thread pilot a plurality of following one another steps having different cross-sections. The thread steps have, respectively, large and small cross-sections, with a thread step with large a cross-section being arranged behind a thread step with a small cross-section, with the thread steps with large and small cross-sections alternating with each other along the screw thread. The thread step with large cross-sections having end surfaces which are formed as cutters and are aligned in the direction of the thread pilot.
The drawback of the fastening element of the German Utility Model consists in that a screw thread with alternatingly arranged thread steps still can cause material spalling during setting of the fastening element.
Accordingly, an object of the present invention is to provide a fastening element with a tapping thread that would produce, in a constitutional component, a counter-thread with essentially almost ideal shape.